Broadband waveguide transition for a centipede slow wave circuit and tubes using same



Oct. 6, 1970 w, DODGE ETAL 3,532,925

BROADBAND WAVEGUIDE TRANSITION FOR A CENTIPEDE SLOW W VE CIRCUIT ANDTUBES USING.SAME

2 SheetS Sheet 1 I Filed May 20, i 1968 BUNCHER CIRCUIT 3 OUTPUT CIRCUITFIG.I

FIG. 2

'mvtmon's EDWARD W. 00005 THEODORE ROUMBANIS BY, EB {1 A ATTORNEY FiledMay 20, 1968 Oct. 6, 1970 w, DODGE EIAL 3,532,925

BROADBAND WAVEGUIDE TRANSITION FOR A CENTIPEDE SLOW WAVE CIRCUIT ANDTUBES USING SAME 2 Sheets-Sheet 2 ROUND COUPLING VSWR HOLE I I/ 2.0 i1.0 l 3132 5.3-3.4 35 :35} an! FIE-3.5

mvEN-ms EDWARD-W. DODGE THEODORE ROUMBANIS ATTORNEY United States PatentU.S. Cl. 315-35 6 Claims ABSTRACT OF THE DISCLOSURE A microwave tube isdisclosed. The microwave tube includes an electron gun for forming andprojecting a beam of electrons over an elongated beam path to a beamcollector at the terminal end of the beam. A centipede type slow wavecircuit is arranged along the beam path for electromagnetic interactionwith the beam to produce an output microwave signal. A broadbandWaveguide transition is provided at the downstream end of the centipedeslow wave circuit for extracting the output microwave signal from thecircuit and for transmitting same to a suitable utilization device orload. The broadband waveguide transition includes an output ridgedrectangular waveguide disposed in wave energy communication with thedownstream end of the centipede slow wave circuit via the intermediaryof a centrally apertured conductive wall partitioning off the downstreamend of the slow wave circuit. A conductive post extends from the ridgedportion of the waveguide into the coupling hole. The coupling hole iselongated in a direction generally perpendicular to the narrow walls ofthe ridged waveguide to provide heavy wave energy coupling from thecentipede slow wave circuit to both the electric and magnetic fields ofthe waveguide to provide a broadband waveguide transition. A conductiveshorting block is provided in the ridged waveguide adjacent the marginallips of the elongated coupling hole for causing the wave energy to bedeflected through a right angle bend between the waveguide and thecoupling hole.

DESCRIPTION OF THE PRIOR ART Heretofore, microwave tubes have beenconstructed which have employed an output slow wave circuit com prisedof an array of cavity resonators having negative mutual inductivecoupling therebetween. Examples of such negative mutual inductivecoupled cavities include the cloverleaf structure and the centipede slowwave structure described and claimed as equivalents in U.S. Pat. 3,233,-139 issued Feb. 1, 1966 and assigned to the same assignee as the presentinvention. Output Waveguide transitions have been developed for suchnegative mutually inductively coupled slow wave circuits. Suchtransitions have included a hollow cylindrical conductive postprojecting from a broad wall of the output waveguide coaxially of thebeam and coaxially of a wave energy coupling hole communicating throughthe end wall of the terminal cavity of the coupled cavity circuit. Morespecifically, the end wall of the terminal cavity of the slow wavecircuit is provided with an enlarged central aperture coaxially alignedwith the beam path. The conductive post extends from a shallow heightrectangular waveguide coaxially of the beam hole and terminatessubstantially in the plane of the coupling hole. The output couplinghole was made approximately /2 a wavelength in diameter and was of acircular shape. The conductive post was cylindrical and the shallowwaveguide was shorted a Wavelength from the center of the conductivepost. Such an output waveguide transition is disclosed and claimed inU.S. Pat.

3,374,390 issued Mar. 19, 1968 and assigned to the same assignee as thepresent invention.

Although the aforedescribed prior art waveguide transition provides anextremely wide band match between a cloverleaf type coupled cavity slowwave circuit and an output Waveguide, such an output transition does nothave sufiicient bandwidth to provide a match to the wider useablebandwidth of a centipede type coupled cavity slow wave circuit.

Therefore a need exists for an improved waveguide transition forcoupling output wave energy from a centipede type coupled cavity slowwave circuit to an output waveguide.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved broadband waveguide transitionfor a centipede slow wave circuit and tubes using same.

One feature of the present invention is the provision, in a broadbandwaveguide transition from a centipede slow wave circuit to an outputwaveguide, of a ridged rectangular output waveguide with a conductivepost extending from the ridge into the mouth of a wave energy couplinghole communicating between the terminal cavity of the slow wave circuitand the waveguide, the output coupling hole being elongated in adirection generally perpendicular to the narrow walls of the outputridged waveguide, whereby heavy wave coupling is obtained to both theelectric and magnetic fields of the output waveguide.

Another feature of the present invention is the same as the precedingfeature wherein a conductive shorting block is provided in the ridgedoutput waveguide, such shorting block being disposed adjacent a portionof the marginal lip of the elongated coupling hole to facilitate abroadband match between the centipede slow wave circuit and the outputwaveguide.

Another feature of the present invention is the same as the precedingfeature wherein the shorting block includes a face which slants into theridged waveguide for deflecting wave energy through a right angle bendbetween the waveguide and the coupling hole.

Another feature of the present invention is the same as the precedingfeature wherein the shorting block includes a curved face conforminggenerally to the adjacent curved surface of the conductive post.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal View, partlyin section and partly schematic, depicting a microwave tubeincorporating features of the present invention,

FIG. 2 is an enlarged longitudinal sectional view of a portion of thestructure of FIG. 1 delineated by line 22,

FIG. 3 is a transverse sectional view of the structure of FIG. 2 takenalong lines 33 in the direction of the arrows,

FIG. 4 is a sectional view of the structure of FIG. 3 taken along line44 in the direction of the arrows, and

FIG. 5 is a plot of voltage standing wave ratio vs. frequency whichcompares the bandpass characteristics of an output waveguide transitionemploying a round energy coupling hole and an oval energy coupling hole,respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a microwave tube 1 incorporating features of the presentinvention. The tube 1 includes an electron gun 2 disposed at one end ofthe tube for forming and projecting a beam of electrons over anelongated beam path 3 to a beam collector structure 4 disposed at theterminal end of the beam path 3. An interaction circuit 5 disposed alongthe beam path intermediate the gun 2 and the collector 4 forelectromagnetic interaction with the beam to produce output microwaveenergy which is extracted from a downstream end of the interactioncircuit 5 via an output waveguide 6 and fed to a suitable utilizationdevice or load, not shown.

The interaction circuit 5 may comprise a single section of centipedetype slow wave circuit or may comprise at least two circuit sections,namely, a buncher circuit section 7 and an output circuit section 8. Inthe latter case, the output circuit section 8 would comprise a centipedeslow wave circuit and the buncher circuit 7 may comprise either asevered section of centipede slow wave circuit or some other type ofinteraction circuit such as, for example, a succession of klystron typecavities tuned near the band edges of the operating band of the tube.Microwave amplifier tubes utilizing a buncher circuit 7 followed by anoutput circuit 8 wherein the output circuit comprises a succession ofnegative mutual inductively coupled cavity resonators is disclosed andclaimed in the aforecited US Pat. 3,374,390.

The centipede type slow wave circuit is one type of negative mutualinductive coupled circuit as disclosed in the aforecited US. Pat.3,233,139. This type of circuit is also described in an article tilted AStructure, Using Resonant Coupling Elements, Suitable for a High-Power Travelling-Wave Tube" by A. F. Pearce, appearing in the Proceedings ofthe Institution of Electrical Engineers, vol. 105, Part B, SupplementII, London, May 1958, pp. 719- 726. The centipede slow wave circuit hasthe advantage of providing extremely wide bandwidth while being capableof operating to very high power levels with reasonable etficiencies.

In operation, an input microwave signal to be amplified is fed to theupstream end of the buncher circuit 7 via input coaxial line 11. Inputsignal energy bunches the beam throughout the buncher circuit 7 and thebunched beam passes into the output circuit 8 wherein it excites a waveon the output circuit which cumulatively interacts with the beam toproduce an amplified output signal. The output microwave signal isextracted from the downstream end of the output circuit 8 and fed viawaveguide 6 and gas tight wave permeable window 12 to a suitable load. Abeam focus solenoid 13 coaxially surrounds the tube to produce anaxially directed beam focusing magnetic field over the length of thebeam path 3 for confining the electrons to the desired beam path.

Referring now to FIGS. 2-4, the novel broadband waveguide transitionfrom the centipede slow wave circuit 8 to the output waveguide 6 will bemore fully disclosed. Briefly, the centipede slow wave circuit 8includes a hollow tubular conductor 14 as of copper having an array ofcentrally apertured conductive disc structures 15 axially spaced apartalong the length of the tube 14 and transversely mounted therein todefine a succession of coupled cavity resonators 16 defined by thespaces bounded by the inside wall of the tube 14 and, on the ends, byadjacent conductive disc structures 15.

The conductive disc structures 15 are conveniently formed by a pair ofsimilarly shaped disc structures 17 and 18, respectively. Each of thedisc structures 17 and 18 includes a central ring portion and an outerperiphery formed by an array of radially directed legs. The two discs 17and 18 are placed together with all of the legs of one of the discsbeing formed into an S shape and all of the legs of the other disc beingformed into a reverse S shape. The two central ring portions of the discstructure are then brazed together. The legs are interdigitated toprovide negative mutual inductive coupling loops communicating betweenadjacent cavity resonators 16. A coolant jacket having coolantpassageways therein surrounds the tube 14 for carrying away heatgenerated in the R.F. structure and which is conducted to the jacket viathe discs 15 and tube 14.

A circular conductive plate 22, as of copper, closes off the downstreamend of the coupled cavity circuit 8. The output waveguide structure 6 isformed in the conductive disc 22 with the axis of the waveguide 6 beingtransverse to the axis of the beam path 3. The waveguide 6 is tapered inheight with decreasing height taken toward the beam axis. In addition,the rectangular waveguide 6 is provided with a central ridge 23 whichprojects from a bottom broad wall of the waveguide toward the upperbroad wall thereof.

A hollow cylindrical conductive post 24, as of copper, surrounds thebeam path 3 and projects from the ridge 23 toward the upper opposedbroad -wall of the guide 6 and also toward the terminal cavity of theslow wave circuit 8. A wave energy coupling hole 25 is provided in theupper wall of the waveguide 6 in coaxial alignment with the beam 3 andthe conductive post 24. Beam coupling hole 25 is elongated in thedirection transverse or perpendicular to the plane of the narrow walls26 of the output waveguide 6. In addition, the coupling hole 25 iscoaxially aligned with the conductive post 24.

The innermost end of the waveguide 6 is closed off by a conductiveshorting plug structure 28 which forms an inner end wall for the ridgedwaveguide 6. The conductive plug structure 28 includes a pair of spacedrectangular plug portions which fill the portions of the ridgedwaveguide 6 between the sidewalls 26 and the center ridge 23. Inaddition, a conductive plate 30 bridges between the pair of rectangularplugs to fill the region of the ridged waveguide between the ridge andthe uppermost wall. The inner shorting face 29 of the two rectangularplug portions of the shorting plug structure 28 is slanted into theguide in such a manner as to deflect wave energy traveling down theguide toward the post 24 through a angle and up through the couplinghole 25. The bridging platelike portion 30 of the conductive plugstructure 28 is cut in an arc shape 31 to conform to the outercylindrical surface of the adjacent conductive post 24. The conductiveplug structure 28 is disposed generally in the region of the lip portionof the cou ling hole 25.

In operation, output wave energy on the centipede slow wave circuit 8reaches the terminal cavity section 16 and is coupled through the outputcoupling hole 25 down along the short section of coaxial line formed bythe post 24 and the adjacent conductive wall structure including theedge of the coupling hole 25. The wave energy then is deflected througha 90 bend, by plug 28, and travels out the ridged waveguide 6 to asuitable load.

The oval shaped coupling hole 25 facilitates obtaining a broad bandtransition between the slow wave circuit 8 and the output waveguide 6.More specifically, the elongated portions of the coupling hole 25, whichextend over the inductive regions of the waveguide, serve to produce amagnetic coupling between the waveguide 6 and the mag netic fields ofthe slow wave circuit 8. On the other hand, the capacitive couplingbetween the post 24 and the regions of closest approach of the marginallip of the hole 25, i.e., the regions along the minor axis of the ovalcoupling hole 25, and the center of the disc structure 15 provide acapacitive coupling between the waveguide 6 and the disc structure 15 ofthe terminal slow wave section 16. This combined electric and magneticcoupling produces a wider band transition than that obtained by use of acircular coupling hole 25.

Referring now to FIG. 5, there is seen a comparison in the voltagestanding wave ratio in the output wavegulde 6 over the passband offrequencies from 3.1 to 3.65 gigahertz for a tube 1 using either a roundcoupling hole 25 or an oval coupling hole 25. Utilizing a round couplinghole 25 it is seen that the voltage standing wave ratio, at the highfrequency end of the passband, reaches the unacceptable level of 2 atapproximately 3.55 gigahertz. However, when an oval coupling hole 25 isemployed the bandwidth between voltage standing wave ratios of 2.0 isextended, at the high frequency end, from approximately 3.55 gigahertzto 3.65 gigahertz.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a microwave tube apparatus, means for forming and projecting abeam of electrons over an elongated beam path, collector means at theterminal end of the beam path for collecting and dissipating energy ofthe beam, a centipede slow wave circuit disposed along the beam path forcumulative electromagnetic interaction between the microwave energy onsaid circuit and electrons of the beam, said centipede slow wave circuitcomprising a hollow conductive tube structure having an array of axiallyspaced centrally apertured conductive discs transversely mounted thereinto define a coupled cavity slow wave structure, said tube and said discsbeing coaxially aligned with the beam path with the beam path passingsuccessively through the aligned central apertures in said discs of saidarray, each of said discs having an array of peripherally spacedradially directed leg portions with adjacent ones of said leg portionsbeing of an S-shape and a reverse S-shape, respectively, to providenegative mutual inductive coupling between adjacent coupled cavities, anoutput coupling means for extracting microwave energy from saidcentipede slow wave circuit for transmission to a load, the improvementwherein, said output coupling means includes an output waveguide havinga pair of narrow walls and a pair of broad walls, a conductive wallclosing off the downstream end of said slow wave circuit and having acentrally disposed coupling hole coaxially aligned with the beam pathand communicating through said wall between said slow wave circuit andsaid waveguide, a hollow conductive post extending into said couplinghole from said waveguide, said coupling hole having a transverse crosssection elongated in a direction generally perpendicular to the narrowwalls of said waveguide whereby, coupling between the magnetic fields insaid slow wave circuit and said waveguide is increased to enhance thebandwidth of said output coupling means.

2. The apparatus of claim 1 wherein said output waveguide is a ridgedrectangular waveguide having a ridge structure projecting from thecenter of one broad wall toward the other broad wall, and wherein saidhollow conductive post projects from said ridge in said waveguide towardthe opposed broad wall of said waveguide, said post axially extendinginto said elongated coupling hole from the ridged portion of said ridgedwaveguide.

3. The apparatus of claim 2 including a conductive shorting block havinga conductive face defining a shorting plane and said shorting blockbeing disposed to close off and to provide a wave energy short for saidridged waveguide, said shorting block being disposed adjacent said postwith its conductive face disposed adjacent a portion of the marginal lipof said elongated coupling hole.

4. The apparatus of claim 3 wherein said shorting face of said shortingblock includes a pair of faces which face into said waveguide from theshorted end thereof, said shorting faces being disposed between thecentral ridge of said waveguide and the narrow walls thereof, saidshorting faces being slanted into said waveguide for deflecting waveenergy through a right angle bend between said waveguide and saidcoupling hole.

5. The apparatus of claim 3 wherein said conductive post is cylindricaland wherein a shorting face portion of said shorting block is curved toconform generally to the adjacent curved surface of said hollowcylindrical conductive post.

6. In a microwave tube apparatus, means for forming and projecting abeam of electrons over an elongated beam path, collector means at theterminal end of the beam path for collecting and dissipating the energyof the beam, a coupled cavity slow wave circuit disposed along the beampath for cumulative electromagnetic interaction between the microwaveenergy on said circuit and the electrons of the beam, an output couplingmeans for extracting microwave energy from said coupled cavity slow wavecircuit for transmission to a load, the improvement wherein said outputcoupling means includes an output waveguide having a pair of narrowwalls and a pair of broad walls, a conductive wall closing otf thedownstream end of said coupled cavity slow wave circuit and having acentrally disposed coupling hole coaxially aligned with the beam pathand communicating through said wall between said slow wave circuit andsaid waveguide, a hollow conductive post extending into said couplinghole from said waveguide, said coupling hole having a transverse crosssection elongated in a direction generally perpendicular to the narrowwalls of said waveguide whereby coupling between the magnetic fields insaid slow wave circuit and said waveguide is increased to enhance thebandwidth of said output coupling means.

References Cited UNITED STATES PATENTS 3,233,139 2/1966 Chodorow 3153.53,374,390 3/1968 Ruetz et al SIS-39.3 3,441,783 4/1969 Harris et al.315-35X HERMAN KARL SAALBACH, Primary Examiner S. CHATMON, JR.,Assistant Examiner US. Cl. X.R.

SIS-39, 39.3; 3333l, 33

